RF: add write0 to seek_tell function and test

Add flag to allow seek_tell to write zeros to reach a given file offset,
if it can't get there with a seek.
BF: test and fix writing of bz2 files with offsets

Writing a bz2 file that wasn't already at the data offset position
caused IOError because of failed write seek on bz2 files.

Use extended seek_tell function to write zeros to reach offset, when
seek fails.
RF: remove redundant _write_header method

We only used image _write_header method for seeking to the data position
in a nifti single image.  Now I've moved that logic to seek_tell, and
therefore 'Analyze.to_file_map', we don't need _write_header.  Remove.
There aren't any good tests for parrec2nii - so I have to hope changes
worked right.
TST: skip test big offset tests without scipy

Offset tests need writing, which need scipy; skip if missing.
Merge pull request #219 from matthew-brett/remove-_write_header

MRG: remove _write_header method

The write_header method in the image classes was only being used to write zero
bytes into a file where the data was not exactly at the end of the header. Use
the seek_tell function for this, and remove the writeheader method from all
classes.